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On a water supply index: Poudre River case study

Date

2010

Authors

Milhous, Robert T., author
Colorado State University, publisher

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Abstract

This paper explores a water supply index calculated from the present year's discharge and discharges in previous years. The paper also compares the use of a water supply index to the use of a single year as a method of classifying annual streamflows into water year classes for the purpose of allocating water to environmental flows. The third task compares the return periods of low annual flows to return periods of a low water supply index. The Cache la Poudre River in Colorado is used as a case study. The water supply index (WSI) is calculated using the equation WSI = {Q(I) + 0.5 Q(I-1) + 0.25 Q(I-2)}/1.75. The index does improve the classification of years into five classes of water years: very dry, dry, moderate dry, moderate wet, and wet. The classification using the water supply index as compared to using annual streamflows only appears to improve the ability of water manages to respond to low flow conditions. The deregulated streamflows in the Cache la Poudre River were analyzed and the 2002 year streamflows were found to have a return period of the annual discharge based on a log-Pearson Type III analysis of 426 years. For the water supply index the return period is 403 years. A long period record has been generated by NOAA using tree rings for 1615-1999. Using a long period record based on tree ring analysis (1615-1999) and deregulated streamflows (2000-2009) the return period for the annual discharge in 2002 is 58 years and 186 years for the water supply index.

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2010 annual AGU hydrology days was held at Colorado State University on March 22 - March 24, 2010.
Includes bibliographical references.

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